Liquid crystal displays have been increasingly becoming popular as displays for information, image and the like. The liquid crystal displays have advantages of providing thin profile, small placing area and the like when compared with CRT (Cathode Ray Tube) displays.
Liquid crystal displays according to the present invention can be used in, for example, a display or monitor for a personal computer, a monitor for factory automation (FA), a home television set, a terminal monitor in a hospital, a library, a museum and the like, a monitor for an air traffic control tower and the like, a monitor used for reference of newspapers, documents in public offices and the like, a personal monitor in a school and a supplementary private school, a personal monitor for utilizing various media, a monitor in amusement facilities such as pachinko facilities, and the like. The liquid crystal displays according to the present invention can also be used as a light valve for projector type liquid crystal displays.
A recent liquid crystal display has image quality substantially equal to that of a CRT display. However, a viewing angle of the liquid crystal display is much narrower than that of the CRT display. Therefore, enlarging a viewing angle is one of the most important concerns in a liquid crystal display.
As a technology for enlarging a viewing angle of a liquid crystal display, there is considered using a liquid crystal display element which has a plurality of domains having different alignment directions in each pixel to obtain a wide viewing angle. Such prior art is disclosed in, for example, Japanese examined patent publication No. 58-43723, “Liquid Crystal Display Element”, Japanese patent laid-open publication No. 59-211019, “Liquid Crystal Display Device”, and Japanese patent laid-open publication No. 63-106624 (Japanese patent publication No. 2692693), “Liquid Crystal Display Panel”.
Referring to a “Liquid Crystal Display Panel” disclosed in Japanese patent laid-open publication No. 63-106624 as a prior art example, among the Japanese publications mentioned above, an explanation will be made on a prior art technology.
FIG. 28 is a plan view showing a conventional liquid crystal display according to this prior art example. FIG. 28 corresponds to FIG. 1 described in the Japanese patent laid-open publication. No. 63-106624. FIG. 29 corresponds to FIG. 2 described in the Japanese patent laid-open publication No. 63-106624. FIG. 30 corresponds to FIG. 3 described in the Japanese patent laid-open publication No. 63-106624. FIG. 30 is a cross sectional view taken on line a-a′ of FIG. 28, and shows a cross section of one pixel of the conventional liquid crystal display device, that is, a liquid crystal display element.
With reference to FIGS. 28 through 30, a structure of the conventional liquid crystal display device will be described.
On a lower glass substrate 122, there are formed a transparent lower display electrode 120 which is provided corresponding to each pixel, and a lower alignment film or layer 110 formed on the transparent lower display electrode 120.
On the lower glass substrate 122, there is also formed a thin film transistor (TFT) 113 for driving the transparent lower display electrode 120.
On an upper glass substrate 121, there are formed a transparent upper display electrode 111 and a upper alignment film or layer 109. The alignment films 109 and 110 are both made of polyimide.
A pixel B formed between the opposing transparent electrodes 111 and 120 has, for example, a square shape of 200 μm (micrometers) square, and a plurality of such pixels B are disposed in a matrix. At the center portion of the transparent display electrode, there is disposed a band shaped spacer 123 made of polyimide. As a result thereof, each pixel B is divided into a region X131 and a region Y132 by the band shaped spacer 123. These divided regions X131 and Y132 are formed as shown schematically in FIG. 16 which corresponds to FIG. 2 of Japanese patent laid-open publication No. 63-106624. That is, the upper glass substrate 121 and the opposing lower glass substrate 122 are respectively rubbed in directions shown by arrows.
In order to show an effect of improvement in viewing angle dependence of viewing quality realized by this prior art liquid crystal display, FIG. 31 shows a plan perspective view which illustrates rubbing directions and direction of twist of liquid crystal alignment between the upper and lower substrates when a direction of observation is changed by 45 degrees in bearing angle. In FIG. 31, there are shown divided regions X131, divided regions Y132, rubbing directions 1311 and 1312 in a first substrate, rubbing directions 1312 and 1322, and a twist angle 127 of liquid crystal alignment between the upper and lower substrates.
FIG. 32 is a cross sectional view taken on line b-b′ of FIG. 31 and showing a rising direction of aligning force at the surface of a substrate and a rising direction of liquid crystal alignment by an electric field between the upper and lower substrates. In FIG. 32, a twist of liquid crystal molecules is not illustrated. In FIG. 32, there are shown liquid crystal material 105, a direction of rising 106 of liquid crystal alignment caused by an electric field, the upper glass substrate 121, a lower glass substrate 122, a region X131, and a region Y132.
In this prior art liquid crystal display, as shown in FIG. 31, with respect to liquid crystal alignment in each of the divided regions or domains, a direction of spiral twist is the same, but as shown in FIG. 32, directions of angles from the substrate surface, i.e., pretilt angles, differ from each other. Because of the difference of the angles from the substrate surface, when a voltage is applied between the upper and lower electrodes, direction 106 of rising up of liquid crystal molecules, which is also called direction of tilt, differ from each other as shown in FIG. 31. Therefore, when light is irradiated from a direction which is inclined from the direction perpendicular to the substrate, optical characteristics of respective domains compensate with each other. As a result thereof, when the voltage is applied, viewing angle dependence is compensated with each other between domains having mutually different alignments in each pixel between the upper and lower substrates, and optical characteristics having small viewing angle dependency can be realized. Especially, when an image having gray shades is displayed, a phenomenon of gray shade inversion is not observed.
Other liquid crystal display panels are also known which function based on the same principle as that of the above-mentioned liquid crystal display panel and which can be fabricated by smaller number of steps than that of the above-mentioned liquid crystal display panel. Such liquid crystal display panels are described in “SID '92 Digest of U.S.A., p. 798”, “Japan Display '92 Digest, p. 591”, “SID '93 Digest of U.S.A., p. 269”, and the like.
In these examples, as in the first example, with respect to alignment of molecules of the liquid crystal material in each of the divided domains, direction of twist of a spiral is the same, but an angle to the substrate surface differs from each other. However, these liquid crystal display panels differ from the first example in a direction of aligning force applied to molecules of the liquid crystal material at the surface of the substrate and magnitude of angle of the aligning force.
In order to clarify the difference between these examples and the first example, perspective plan views and cross sectional views for each of these examples. The perspective plan views show directions of rubbing and directions of twist of alignment of liquid crystal molecules between upper and lower substrates. The cross sectional views show directions of rising up of the alignment force at the surface of the substrate and directions of rising up of alignment of liquid crystal between the upper and lower substrates caused by an electric field.
FIG. 33 is a plan perspective view showing directions of rubbing and directions of twist of alignment of liquid crystal molecules between upper and lower substrates, in the prior art example shown in “SID '92 Digest of U.S.A., p. 798”. FIG. 34 is a cross sectional view taken on line c-c′ of FIG. 33, and shows directions of rise of the aligning force at the surface of the substrate and directions of rise of alignment of liquid crystal molecules between the upper and lower substrates caused by an electric field.
An explanation will be made on the liquid crystal display element shown in FIG. 33 and FIG. 34. In the plan perspective view of FIG. 33, there are shown a divided domain X131, a divided domain Y132, a rubbing direction 1311 in the domain X on an upper glass substrate, a rubbing direction 1312 in the domain X on a lower glass substrate, a rubbing direction 1321 in the domain Y on the upper glass substrate, a rubbing direction 1322 in the domain Y on the lower glass substrate, and twist angle 127 of the liquid crystal alignment. Also, in the cross sectional view of FIG. 34, there are shown the upper glass substrate 121, the lower glass substrate 122, the divided domain X131, the divided domain Y132, liquid crystal material 105, direction 106a of rise of alignment of liquid crystal molecules caused by an electric field in the domain X, and direction 106b of rise of alignment of liquid crystal molecules caused by an electric field in the domain Y.
In this example, rubbing direction of the upper substrate is one direction, i.e., the same direction, in both of the divided domains, and rubbing direction of the lower substrate is one direction, i.e., the same direction, in both of the divided domains. However, materials of the upper and lower substrates are changed between the domain X and the domain Y. Therefore, as shown in FIG. 34, in the domain X131, the angle of liquid crystal molecules from the surface of the lower substrate 122 is higher (larger) than the angle of liquid crystal molecules from the surface of the upper substrate 121. In the domain Y132, the angle of liquid crystal molecules from the surface of the lower substrate 122 is lower (smaller) than the angle of liquid crystal molecules from the surface of the upper substrate 121. Therefore, in the domain X131, the direction 106a of rise of liquid crystal alignment caused by an electric field becomes a direction of aligning force at the lower glass substrate 122, when a voltage is applied to this liquid crystal display element. In this case, in the domain Y132, the direction 106b of rise of liquid crystal alignment caused by an electric field becomes a direction of aligning force at the upper glass substrate 121.
FIG. 35 is a plan perspective view showing directions of rubbing and directions of twist of alignment of liquid crystal molecules between upper and lower substrates, in the prior art example shown in “Japan Display '92 Digest, p. 591”. FIG. 36 is a cross sectional view taken on line d-d′ of FIG. 35, and shows directions of rise of the aligning force at the surface of the substrate and directions of rise of alignment of liquid crystal molecules between the upper and lower substrates caused by an electric field.
An explanation will be made on the liquid crystal display element shown in FIG. 35 and FIG. 36. In the plan perspective view of FIG. 35, there are shown a divided domain X131, a divided domain Y132, a rubbing direction 1311 in the domain X on an upper glass substrate, a rubbing direction 1312 in the domain X on a lower glass substrate, a rubbing direction 1321 in the domain Y on the upper glass substrate, a rubbing direction 1322 in the domain Y on the lower glass substrate, and twist angle 127 of the liquid crystal alignment. Also, in the cross sectional view of FIG. 36, there are shown the upper glass substrate 121, the lower glass substrate 122, the divided domain X131, the divided domain Y132, liquid crystal material 105, direction 106 of rise of alignment of liquid crystal molecules caused by an electric field.
In this example, rubbing direction of the upper substrate is one direction, i.e., the same direction, in both of the divided domains, and rubbing directions of the lower substrate have two different directions. Also, as shown in FIG. 36, in both the domain X131 and the domain Y132, the angle of liquid crystal molecules from the surface of the lower substrate 122 is higher (larger) than the angle of liquid crystal molecules from the surface of the upper substrate 121. However, in the lower glass substrate 122, directions of aligning force differ between the domain X131 and the domain Y132. Therefore, in both the domain X131 and the domain Y132, the direction 106 of rise of alignment of liquid crystal molecules caused by an electric field becomes a direction of aligning force at the lower glass substrate 122, when a voltage is applied to this liquid crystal display element.
FIG. 37 is a plan perspective view showing directions of rubbing and directions of twist of alignment of liquid crystal molecules between upper and lower substrates, in the prior art example shown in “SID '93 Digest of U.S.A., p. 269”. FIG. 38 is a cross sectional view taken on line e-e′ of FIG. 37, and shows directions of rise of the aligning force at the surface of the substrate and directions of rise of alignment of liquid crystal molecules between the upper and lower substrates caused by an electric field.
An explanation will be made on the liquid crystal display element shown in FIG. 37 and FIG. 38. In the plan perspective view of FIG. 37, there are shown a divided domain X131, a divided domain Y132, a rubbing direction 1311 in the domain X on an upper glass substrate, a rubbing direction 1312 in the domain X on a lower glass substrate, a rubbing direction 1321 in the domain Y on the upper glass substrate, a rubbing direction 1322 in the domain Y on the lower glass substrate, and twist angle 127 of the liquid crystal alignment. Also, in the cross sectional view of FIG. 38, there are shown the upper glass substrate 121, the lower glass substrate 122, the divided domain X131, the divided domain Y132, liquid crystal material 105, direction 106 of rise of alignment of liquid crystal molecules caused by an electric field.
In this example, rubbing direction of the upper substrate is one direction, i.e., the same direction, in both of the divided domains, and rubbing direction of the lower substrate is also one direction, i.e., the same direction, in both of the divided domains. Also, in both the domain X131 and the domain Y132, the angle of liquid crystal molecules from the surface of the lower substrate 122 is the same as the angle of liquid crystal molecules from the surface of the upper substrate 121. However, in each of the domain X131 and the domain Y132, directions of aligning force differ between the upper substrate 121 and the lower substrate 122. When no voltage is applied to this liquid crystal display element, alignment of liquid crystal molecules at the central portion is held horizontal. When a voltage is applied to this liquid crystal display element, the direction 106 of rise of alignment of liquid crystal molecules is determined by electric field having slant directions as shown in FIG. 38.
In these three prior art examples, by adopting structural improvement as shown in FIG. 33 through FIG. 38, the number of process steps is decreased from that of the above-mentioned prior art. In any of these methods, as shown in FIG. 34, FIG. 36 and FIG. 38, when a voltage is applied, directions of rise of liquid crystal molecules differ from each other in respective domains. Therefore, when light is irradiated from a direction slanted from the normal of the substrate, optical characteristics of both regions are mutually compensated. As a result thereof, viewing angle dependency in the condition a voltage is applied is mutually compensated between regions having different alignments in each pixel between the upper and lower substrates, so that optical characteristics having reduced viewing angle dependency.
On the other hand, there is a commonly known thin film transistor (TFT) type liquid crystal display which uses usual twisted nematic (TN) type liquid crystal display mode. Also, among such commonly known liquid crystal display, there is a liquid crystal display in which liquid crystal alignment is controlled by forming a slit or slits in a pixel electrode of an opposing substrate. Such liquid crystal display is disclosed in Conference Record of the 1991 International Display Research Conference, p. 239 and an explanation will be made on such liquid crystal display.
FIG. 39, which corresponds to FIG. 1(b) of the above-mentioned document, shows a cross sectional view of a part, i.e., one pixel, of the liquid crystal display disclosed in the above-mentioned document. FIG. 40, which corresponds to FIG. 7(a) of the above-mentioned document, illustrates electric field in lateral direction in such liquid crystal display. Also, FIG. 41, which corresponds to FIG. 1(a) of the above-mentioned document, shows a cross sectional view of a part, i.e., one pixel, of the commonly known liquid crystal display which uses usual TN type liquid crystal display mode. FIG. 29, which corresponds to FIG. 7(b) of the above-mentioned document, illustrates electric field in such liquid crystal display.
First, an explanation will be made on the liquid crystal display shown in FIG. 39 and FIG. 40. In the cross sectional view of FIG. 39, there are shown an upper glass substrate 221, a lower glass substrate 222, a signal electrode line 215, a lower transparent electrode, that is, a pixel electrode, 220, an upper transparent electrode, that is, an opposing electrode 219, a black mask 231, a color layer 232, an overcoat 233, a lower silicon nitride film 204, and an upper silicon nitride film 241.
On the upper glass substrate 221, there are disposed, on the side facing toward the lower glass substrate 222, the black mask 231 for shutting out the light, the color layer 232 for passing the light of predetermined color, the overcoat 233 for protecting the black mask 231 and the color layer 232, and the opposing electrode 219.
On the lower glass substrate 222, there are disposed, on the side facing toward the upper glass substrate 221, the signal electrode line 215, the lower silicon nitride film 204 for covering the lower glass substrate 222 and the signal electrode line 215, the pixel electrode 220 formed on the lower silicon nitride film 204, and the upper silicon nitride film 241 covering whole portion on the lower glass substrate 222.
In FIG. 40, electric field 202 is shown by arrows which is produced when a voltage is applied to the liquid crystal display element having the structure shown in FIG. 39.
Next, an explanation will be made on the commonly known TN type liquid crystal display element shown in FIG. 41 and FIG. 42. In the cross sectional view of FIG. 41, there are shown an upper glass substrate 221, a lower glass substrate 222, a signal electrode line 215, a lower transparent electrode, that is, a pixel electrode, 220, an upper transparent electrode, that is, an opposing electrode 229, a black mask 231, a color layer 232, an overcoat 233, a lower silicon nitride film 204, and an upper silicon nitride film 241.
On the upper glass substrate 221, there are disposed, on the side facing toward the lower glass substrate 222, the black mask 231 for shutting out the light, the color layer 232 for passing the light of predetermined color, the overcoat 233 for protecting the black mask 231 and the color layer 232, and the opposing electrode 229.
On the lower glass substrate 222, there are disposed, on the side facing toward the upper glass substrate 221, the signal electrode line 215, the lower silicon nitride film 204 for covering the lower glass substrate 222 and the signal electrode line 215, the pixel electrode 220 formed on the lower silicon nitride film 204, and the upper silicon nitride film 241 covering whole portion on the lower glass substrate 222.
In FIG. 42, electric field 202 is shown by arrows which is produced when a voltage is applied to the liquid crystal display element having the structure shown in FIG. 41.
In the prior art liquid crystal display element shown in FIG. 39 and FIG. 40, disturbance of liquid crystal alignment, which is called disclination and which is caused by lateral electric field produced between a pixel electrode and a wiring electrode such as a scanning electrode line, a signal electrode line and the like, can be reduced.
Here, the first problem in the above-mentioned prior art liquid crystal display element is that disturbance of alignment which is called disclination and which is caused by lateral electric field between the wiring electrode and the pixel electrode. This is because, potential of the wiring electrode such as the scanning electrode line, the signal electrode line and the like gives large influence on the electric field. Thereby, relatively large lateral electric field is produced and liquid crystal alignment is changed by such lateral electric field. In order to reduce the disclination, it was necessary to provide a slit in the opposing electrode by patterning it.
The second problem in the above-mentioned prior art liquid crystal display element is that, when pressure is applied from the outside to the liquid crystal display panel having such liquid crystal display element by pressing the panel by a finger or by a pen point, alignment of liquid crystal molecules is easily disturbed, or alignment division collapses or becomes defective.
This is because, the prior art liquid crystal display panel does not have sufficient mechanical strength and cell gap of the liquid crystal display panel changes easily by the pressure applied thereto, so that alignment of the liquid crystal molecules is changed.
Also, it is impossible to easily and simply realize division of a pixel region into a plurality of domains having different alignment to obtain a wide viewing angle.
There are other Japanese patent applications each relating to realizing a wide viewing angle in a liquid crystal display.
One of such applications is Japanese patent laid-open publication No. 7-104282 and entitled “Liquid Crystal Display and Method of Manufacturing the Same”. This application provides a liquid crystal display and a method of manufacturing the same in which a liquid crystal portion is divided or partitioned by high polymer material into a plurality of liquid crystal regions, and in which liquid crystal molecules or high polymer material in the liquid crystal regions are driven to various modes depending on the alignment condition, or to a mode in which the liquid crystal region is aligned into symmetry with respect to an axis.
This liquid crystal display comprises a plurality of liquid crystal regions each of which has an electrode, is disposed between a pair of substrates, at least one of the substrates being transparent, and is surrounded by the high polymer material. This liquid crystal display also comprises a light control layer which is disposed between the surface of at least one substrate and an electrode formed on the substrate, which controls an intensity of incident light by changing transmittance and which has locally different film thickness. Also, the space between the electrodes on respective surfaces of the pair of substrates varies corresponding to the change of the film thickness of the light control layer. By such structure, mechanical strength is improved, and also viewing characteristic, contrast and color reproductivity are improved.
Another one of such applications is Japanese patent laid-open publication No. 7-333612 and entitled “Liquid Crystal Display Device”. This application provides a liquid crystal display device having a wide viewing angle by averaging angle dependency of optical transmittance. The liquid crystal display device disclosed in this application has a pair of opposing substrates each of which comprises a transparent electrode formed on the surface thereof and an alignment film formed on the transparent electrode, and a liquid crystal layer of liquid crystal molecules filling the space between the substrates. When a voltage is not applied to the transparent electrodes, in a plane perpendicular to the direction of the thickness of the liquid crystal layer, there are a first liquid crystal domain in which direction of alignment of liquid crystal molecules is approximately constant and a second liquid crystal domain in which direction of alignment of liquid crystal molecules differs from that of the first liquid crystal domain. These two liquid crystal domains are realized by providing uneven portions at alignment layer formed on at least one of the substrate.
In a conventional liquid crystal display of a twisted nematic (hereafter referred to as “TN”) type which is widely used, when an electric field is not applied to liquid crystal molecules, a direction of an alignment vector of the liquid crystal molecules is parallel to a surface of a substrate and this condition corresponds to the condition in which “white” is displayed. When a voltage is applied to the liquid crystal display, the direction of the alignment vector varies toward a direction of an electric field according to a voltage applied. Thereby, the condition in which “white” is displayed is gradually changed to the condition in which “black” is displayed.
However, because of behavior peculiar to liquid crystal molecules in a condition a voltage is applied, there occurs a problem that the TN type liquid crystal display has a narrow viewing angle. This problem of narrow viewing angle is prominent in a direction toward which liquid crystal molecules rise when medium gray is displayed.
As a method of improving viewing angle characteristics, there are proposed some liquid crystal displays disclosed in Japanese patent laid-open publication No. 4-261522, Japanese patent laid-open publication No. 6-43461 and Japanese patent laid-open publication No. 10-333180.
In these liquid crystal displays, viewing angle characteristics are improved by using a structure shown in FIG. 44A, FIG. 44B and FIG. 44C. A liquid crystal display panel of the structure shown in these drawings comprises an upper substrate having a color filter substrate 501 and having a common electrode 502 and an alignment layer 503 which are formed on the color filter substrate and which have a slit 517 formed in the common electrode 502 and the alignment layer 503. Also, there is provided a lower substrate having a substrate 507, a pixel electrode 504 formed on the substrate 507 and an alignment layer 503 formed on the pixel electrode 504. A space between the upper substrate and the lower substrate is filled with liquid crystal molecules 508 which are homeotropic aligned and which form a liquid crystal layer. The liquid crystal display panel shown in FIG. 44A, FIG. 44B and FIG. 44C is put between a pair of polarizers disposed such that polarizing axes or transmission axes of these polarizers have right angles with each other. When a voltage is applied between the common electrode 502 and the pixel electrode 504, an electric field having slant components is produced. Therefore, more than one liquid crystal domains having different alignment directions are produced in each pixel, and thereby viewing angle characteristics are improved. The shape of the slit can be, for example, a rectangle as shown in FIG. 44B, or can be x-shape as shown in FIG. 44C.
In the liquid crystal display disclosed in Japanese patent laid-open publication No. 4-261522, directions of inclination of liquid crystal molecules when an electric field is applied thereto are controlled to obtain a high contrast ratio. Also, as disclosed in Japanese patent laid-open publication No. 6-43461, it is possible to use an optical compensation film to improve viewing angle characteristics of black area.
Further, in Japanese patent laid-open publication No. 6-43461, not only in a liquid crystal display panel having homeotropic alignment, but also in a liquid crystal display panel having TN alignment, each pixel is divided into more than one domains by using an electric field having slant and different components to improve viewing angle characteristics.
In Japanese patent laid-open publication No. 10-333180, a technology is disclosed in which thin film transistors, gate lines and drain lines are disposed under display electrodes, in order to prevent an electric field from the thin film transistors, the gate lines and the drain lines from affecting slant electric field components produced by using a common electrode having opening portions.
In Japanese patent laid-open publication No. 10-20323, there is disclosed a liquid crystal display in which more than one kind of minute domains coexist in a liquid crystal layer. One of substrates of liquid crystal display panel of this liquid crystal display has opening portions, and there is provided a second electrode in each of the opening portions. A voltage is applied to the second electrode to produce an electric field having slant components. Thereby, domains having different alignment directions are produced in each pixel and wide viewing angle characteristics are obtained. This publication mainly concerns a liquid crystal display having TN type cell.
Japanese PCT patent laid-open publication No. 5-505247 discloses a liquid crystal display of In-Plain-Switching (IPS) system or mode in which, in order to rotate liquid crystal molecules while keeping the liquid crystal molecules in a direction parallel to substrates of liquid crystal display panel, two electrodes are disposed on one of the substrates. A voltage is applied to between the two electrodes and, thereby, an electric field parallel to the substrates is produced. In such IPS system, a long axis of each of liquid crystal molecules does not rise from the substrate when the voltage is applied. Therefore, when a viewing angle is changed, variation of birefringence or double refraction of liquid crystal is relatively small and, therefore, wide viewing angle characteristics are obtained.
Also, in each of “Journal of Applied Physics”, Vol. 45, No. 12 (1974) 5466 and Japanese patent laid-open publication No. 10-186351, there is disclosed a liquid crystal display which has the above-mentioned IPS mode and in which liquid crystal molecules having a positive anisotropy of permittivity are aligned as homeotropic alignment. The liquid crystal molecules are rotated toward a direction parallel to the substrate by using an electric field parallel to the substrate. In this case, because of directions of the electric field, the liquid crystal molecules which are homeotropic aligned are grouped into two domains. The two domains have different alignment directions of liquid crystal molecules and, therefore, it is possible to obtain a liquid crystal display having wide viewing angle characteristics.
Japanese patent laid-open publication No. 10-186330 proposes a liquid crystal display in which square wall structures are produced by using a photosensitive material, and a pixel is formed by using each of such structures as a basic unit. When a voltage is applied to produce an electric field, liquid crystal molecules having a negative anisotropy of permittivity are rotated toward different directions in a pixel.
However, in the above-mentioned technology in which slits are provided in the common electrode, it is necessary to perform a microfabrication process, such as photolithography and the like, of the common electrode 502 which is not required in the fabrication process of the usual TN type liquid crystal display. In addition to this, there is a problem that a very precise assemble technology is required for precisely combine the upper substrate 501 and the lower substrate 507. This problem is especially important in an active matrix type liquid crystal display which uses switching elements such as TFT's and the like.
That is, in a usual active matrix type liquid crystal display, active elements such as thin film diodes and the like are formed on one of transparent substrates. Therefore, it is only necessary to perform a microfabrication process, such as photolithography and the like, on one of the substrate on which active elements are formed. It is not necessary to perform such microfabrication process on the other substrate which is usually called a “common electrode”, but it is only necessary to form an electrode on whole area of the other substrate.
However, in the above-mentioned prior art technologies, it is also necessary to perform a microfabrication process, such as photolithography and the like, on the “common electrode” on which, usually, it is not necessary to perform such microfabrication process. Therefore, fabrication process becomes complicated, and a high precision alignment and assemble technology is required for the upper substrate 501 and the lower substrate 507.
When the thin film transistors, the gate lines and the drain lines are disposed under the display electrodes, as described in Japanese patent laid-open publication No. 10-333180, an aperture ratio becomes deteriorated.
Also, in the liquid crystal display described in Japanese patent laid-open publication No. 10-20323, it is necessary to apply a voltage to each of second electrodes when driving the liquid crystal display and, therefore, a special drive technology is required. Further, an additional process is required in which a voltage is applied to the second electrodes in order to obtain divided alignment directions.
In the IPS type liquid crystal display and in the liquid crystal display in which vertically aligned liquid crystal molecules are rotated down by using the lateral electric field, there is a problem that an aperture ratio is deteriorated. Also, in these liquid crystal displays, when a cell gap of each pixel is narrowed to obtain high operating speed, a high drive voltage is required.
In the IPS type liquid crystal display and in the liquid crystal display in which vertically aligned liquid crystal molecules are rotated down by using the lateral electric field, there is still another problem as follows. That is, in such prior art liquid crystal displays, since a color filter layer is disposed between a layer in which liquid crystal molecules are disposed and an opposing substrate, especially when switching elements are formed by using a TFT structure, an electric field produced by applying a voltage between the source electrode and the common electrode affects the color filter layer, and deteriorates display characteristics.
That is, in coloring matter constituting the color filter layer, sodium ions are included as impurities. Therefore, when an electric field is applied to the color filter, electric charges are stored in the color filter layer and the color filter layer is charged up. If the color filter layer is charged up, an unnecessary electric field is always applied to the liquid crystal molecules under the charged up portion of the color filter layer. Therefore, such electric field affects display characteristics, especially color irregularity.
In the liquid crystal display which has square walls disclosed in Japanese patent laid-open publication No. 10-186330, it is necessary to fabricate the walls by using photolithography, in order to realize divided alignment directions. Therefore, a manufacturing process becomes complicated.